Although the notion that behavioral and autonomic thermoregulatory functions may be controlled by distinct central structures has been supported experimentally, results to-date do not permit as yet a coherent description of the relevant networks. In particular, the locations and interconnections of the brain regions mediating the pattern of behavioral and autonomic thermoregulatory responses associated with fever production remain to be determined. The present, proposed experiments are directed toward elucidating these relationships. The approach will be: (1) to microinject leukocytic pyrogen (LP) directly into those brain sites previously demonstrated to evoke autonomic febrile responses when so stimulted, as well as into those sites shown earlier to elicit behavioral thermoregulatory responses (but not as yet to be sensitive to LP), and determine from the observed responses which among those will evoke a behavioral fever; then, (2) to identify the efferent pathways from the LP-sensivite sites to the relevant thermoeffector areas, by servering with microknives discrete fiber tracts and measuring the resulting changes in individual components of autonomic and behavioral thermoregulation. Adult and newborn guinea pigs will be used in these studies, the latter age group to determine whether behavioral fevers occur ontogenetically before autonomic ones, as has recently been alleged. The results are expected to contribute knowledge toward increasing our understanding of the organization of the hypothalamus for thermal homeostatis as well as shed light on the ontogeny of this process.